The present invention relates generally to a method for preparing carbonless paper in which particles of a hot melt composition are applied to the surface of pressure sensitive carbonless transfer sheets to prevent the microcapsules from being prematurely or inadvertently ruptured. In accordance with the preferred embodiment of the invention, the particles are applied using an ink jet.
Hot melt inks have been used in ink jet printing applications where high quality color prints having sharply defined images of high resolution and superior edge definition are desired. See, e.g., U.S. Pat. No. 5,223,860 to Lofbourow et al; U.S. Pat. No. 5,350,446 to Lin et al; U.S. Pat. No. 5,409,530 to Kamboyashi et al; U.S. Pat. No. 5,514, 209 to Larson, Jr.; and U.S. Pat. No. Re. 34,029 to Ball. Brown et al. in U.S. Pat. No. 5,185,035 teaches the use of a transparent hot melt inks to make color reproductions on paper or overhead projection transparencies where highly transparent colored inks are necessary.
Hot melt materials also have been used in carbonless coating compositions as a vehicle or suspending medium for pigment particles and/or encapsulated chromogenic materials as discussed in commonly assigned U.S. Pat. Nos. 4,097,619; 4,139,392; 4,143,890; and 4,162,165.
Carbonless transfer sheets, to which the present invention is particularly directed, have been commercially available for many years. One type of carbonless transfer sheet is known as a CB (coated back) sheet. The backside of a CB sheet is coated with a layer of microcapsules. CB sheets are assembled with sheets known as CF sheets in which the front side of the sheet is coated with a composition containing one or more color developers. When a CF sheet and a CB sheet are assembled so that the CB coating faces the CF coating, and sufficient pressure is applied in a predetermined configuration, such as by writing, the capsules in the CB sheet rupture and release the color precursor which is then transferred to the CF sheet where it reacts with the color developer to form an image. Another sheet known as a CFB sheet is coated on one side with microcapsules and the other side with developer and can be interleaved between a CB and a CF sheet to provide a multipart form.
Another type of carbonless transfer sheet is a self-contained paper in which only one side of the paper is coated with both the color precursor, in encapsulated form, and the color developer in a single coating. Thus, when pressure is applied, the color precursor capsule is ruptured and color precursor is released where it reacts with the surrounding color developer to form an image on an adjacent imaging sheet. As can be appreciated, both of these forms of carbonless sheets are extremely sensitive to minute pressure and subject to premature random development during routing handling of the sheets.
In order to prevent premature and inadvertent rupture of the microcapsules during manufacture and handling, it is common practice to incorporate stilt particles such as starch granules into the capsule coating mixture prior to its application to the paper. See for example, U.S. Pat. No. 4,931,920 to Asamo et al., in which stilt materials such as wheat starch, potato starch, cellulose starch and various synthetic resin particles are used to prevent microcapsules from being broken. The stilt particles, being larger than the capsules, protect the capsules from being prematurely ruptured by pressures associated with normal handling of the sheets and manufacturing operations.